Hyundai Sante Fe (2009) - I have an intermittent problem whereby engine light comes on and I can only drive at 40-50kmh max. on flat road, less on inclines

Problem happened once last March after a 25-minute drive and being parked for about 10 mins. Started car, engine light was on and limited acceleration - 40 kmh.  Drove to garage, computer read that it was a 'throttle' problem.   Rectified itself overnight.  Didin't recur until last December and again rectified itself after an overnight rest.   This past month it has happened four times, twice after being parked for a short period of time (once at the carwash) but the last times, it happened on the run.   Had a couple of things done which I think consisted of cleaning a part (maybe EGR Valve) and some re-programming.   My mechanic conferred with the local Hyundai dealer and both seem to agree that it can't be the 'throttle' b/c the problem is only intermittent.   However after $400 and now being afraid to drive outside city limits, on any road that has a steep incline or over any bridges, I'm stuck with a car that is unreliable and an unsolved mystery.   Any ideas?